A known conventional technology disclosed, for instance, by Japanese Patent Laid-Open No. 2004-251273 supplies a mixture of hydrocarbon fuel and air to a catalyst, obtains reformed gas through a reforming reaction with the catalyst, and supplies the obtained reformed gas to an internal combustion engine. A fuel reforming apparatus described in Japanese Patent Laid-Open No. 2004-251273 uses a partial oxidation reaction as a reforming reaction. When hydrocarbon fuel is subjected to partial oxidation, a reformed gas containing H2 and CO is generated as indicated in the following chemical formula:CmHn+(m/2)O2−>mCO+(n/2)H2  (1)
Another known fuel reforming apparatus adds steam to a mixture of hydrocarbon fuel and air, supplies the resulting mixture to a catalyst, and obtains reformed gas. In this instance, the hydrocarbon fuel is subjected to a steam reforming reaction with the catalyst as indicated in the following chemical formula in addition to the aforementioned partial oxidation reaction:CmHn+mH2O−>mCO+(m+n/2)H2  (2)
H2 and CO, which are generated as a result of the above partial oxidation reaction and steam reforming reaction, excel in combustibility. Therefore, when, for instance, a reformed gas containing H2 and Co is supplied to an internal combustion engine at the time of a cold start, the startability of the internal combustion engine can be improved. In addition, the exhaust emission quality can also be improved.
Including the above-mentioned document, the applicant is aware of the following documents as a related art of the present invention.
[Patent Document 1]
Japanese Patent Laid-Open No. 2004-251273
[Patent Document 2]
Japanese Patent Publication No. Hei5-65708
[Patent Document 3]
Japanese Patent Laid-Open No. 2001-227419
[Patent Document 4]
Japanese Patent Laid-Open No. 2000-7303
[Patent Document 5]
Japanese Patent Laid-Open No. Hei8-91802
[Patent Document 6]
Japanese Patent Laid-Open No. Hei6-219701
The reaction speed of the aforementioned partial oxidation reaction is high. When the air-fuel mixture flows to the catalyst, the reaction virtually terminates in an upstream area of the catalyst. FIG. 4 is a graph illustrating the relationship between the catalyst bed temperature and the position within the catalyst in the direction of a gas flow. As indicated in the graph, the catalyst bed temperature is extremely high in an upstream area of the catalyst in which a partial oxidation reaction (PO reaction) has progressed. The reason is that the partial oxidation reaction is an exothermic reaction. The catalyst is heated by reaction-induced heat. On the other hand, in a downstream area of the catalyst in which the partial oxidation reaction is virtually terminated, the catalyst bed temperature gradually lowers due to heat dissipation from the catalyst. Further, CO2 and H2O are generated in a lean region as well as H2 and CO due to fuel atomization failure or mixing failure, which occurs during the use of an air-fuel mixture. In a rich region, on the other hand, unreformed HC is generated. The generated CO2, H2O, and unreformed HC react in a downstream area of the catalyst as indicated in the following reaction formula:aCmHn+bCO2+cH2O−>dCO+eH2  (3)
Since the above reaction is an endothermic reaction, the catalyst bed temperature in a downstream area of the catalyst further decreases.
The steam reforming reaction (SR reaction) is faster in reaction speed than the partial oxidation reaction. Therefore, when a mixture containing hydrocarbon fuel, air, and steam flows to the catalyst, mainly the partial oxidation reaction occurs in an upstream area of the catalyst, and mainly the steam reforming reaction occurs in a downstream area of the catalyst. FIG. 5 is a graph illustrating the relationship between the catalyst bed temperature and the position within the catalyst in the direction of a gas flow. As indicated in the graph, the catalyst bed temperature is extremely high in an upstream area of the catalyst in which a partial oxidation reaction, that is, an exothermic reaction, has progressed. On the other hand, in a downstream area of the catalyst in which the steam reforming reaction has progressed, the catalyst bed temperature significantly lowers. The reason is that heat is released from the catalyst due to the progress of the steam reforming reaction, which is an endothermic reaction, in addition to heat dissipation from the catalyst.
As described above, an upstream area of the catalyst in the conventional fuel reforming apparatus is readily overheated by the heat generated by a partial oxidation reaction, and the catalyst bed temperature in a downstream area of the catalyst readily lowers due to heat dissipation and endothermic reaction such as steam reforming reaction.
However, when the catalyst is excessively overheated, the precious metal in the catalyst may deteriorate due to sintering. Further, if the honeycomb structure that supports the catalyst is made of metal, it may corrode due to high-temperature oxidation. Even when a ceramic honeycomb structure is used; its strength may decrease. An outer casing may also corrode due to high-temperature oxidation because they are made of metal.
Meanwhile, when the catalyst bed temperature lowers in a downstream area of the catalyst, the concentrations of H2 and CO in the reformed gas decrease to increase the concentration of THC. This is caused by the following methane generation reaction that progresses when the catalyst bed temperature lowers:2H2+2CO−>CO2+CH4  (4)
When the above reaction progresses, the concentrations of H2 and CO in the reformed gas decrease to increase the concentration of CH4. A graph in FIG. 6 shows the relationship between the catalyst bed temperature and the THC concentration in the reformed gas. As indicated in the graph, there is an appropriate catalyst bed temperature that minimizes the THC concentration. The THC concentration increases as the catalyst temperature decreases from the appropriate temperature.